This invention relates to a selective excitation method of nuclear spins inside a patient in a magnetic resonance imaging for obtaining a tomogram of the patient by utilizing nuclear magnetic resonance (NMR) and an MRI apparatus using the method. More particularly, the present invention relates to an art of removing artifacts on the tomogram resulting from admixture of signals from portions of a patient positioned outside a homogeneous static magnetic field space region into signals inside a slice plane.
An MRI apparatus must separate NMR signals from an inspection object to signals corresponding to respective positions inside the inspection object and must identify them, respectively. To this end has been employed a method which makes the intensity of a magnetic field, in which each portion of the object is placed, different from others so as to make different a resonance frequency of a nuclear spin inside the inspection object and its dephase quantity, and picks up detection signals after position data are applied to them.
To obtain a tomogram of a human body in an arbitrary direction, the MRI apparatus uses a magnet for generating a static magnetic field having an intensity as high as from 0.05 to 2 Tesras with homogeneity of some dozens of parts-per-million (ppm) in a broad spherical space region having a diameter of as great as from 30 to 50 cm. Unlike the homogeneous magnetic field region described above, however, the magnetic field intensity drops or rises and becomes heterogeneous in the space outside the homogeneous magnetic field region of the 30-50 cm spherical space.
Since the homogeneous magnetic field space inside the static field generation magnet exists only inside the spherical space having a diameter of 30 to 50 cm as described above, a certain portion or portions of the patient inevitably protrude from the homogeneous magnetic field space when the patient is placed into the space of the static magnetic field generation magnet for imaging.
If a pulse sequence of applying a radio frequency (RF) pulse of a frequency f.sub.1 is executed for imaging under this state, not only a nuclear spin inside an imaging plane, i.e. a slice plane 51 of the patient, but also a nuclear spin inside a certain plane 52 of the patient outside the homogeneous magnetic field space are excited simultaneously and selectively when the nuclear spin inside the slice plane 51 is excited selectively as shown in FIG. 1. Accordingly, the signal from this outside portion is detected simultaneously with the signal from inside the slice plane, appears as an artifact on an image reconstructed, and reduces image quality of the resulting tomogram.
Recognition of the existence of this problem and one of the methods of solving the problem are already disclosed in JP-B-3-74100. Therefore, though the explanation in detail of the causes for the occurrence of artifacts and the method of solving them are hereby omitted, the method disclosed in this reference leaves the following problems yet to be improved.
The MRI apparatus has an excellent advantage that a tomogram of a patient in an arbitrary direction can be obtained noninvasively, but involves the problem that an imaging time is long. However, the technique disclosed in the reference described above must add a pulse sequence portion for removing artifacts to an original imaging pulse sequence, so that the imaging time becomes further elongated. The elongated time itself is generally in the order of 10 to 20 ms for measurement of the imaging time of one slice plane. However, if this method is applied to a multi-slice measuring method used frequently in the MRI apparatus and if imaging is to be completed within a predetermined time, the number of slices must be decreased. The proportion of this decrease is about 20% to 40%, though depending on the pulse sequence for imaging. Therefore, the measurement cannot be finished by a single measurement but must be repeated twice, so that a processing capacity of a patient (through-put) drops.
The second problem is as follows. According to the method of the reference described above, the nuclear spin of the portion of the patient outside the homogeneous static magnetic field region is excited without applying the gradient magnetic field prior to the pulse sequence for imaging. However, since such a portion has generally a greater volume than the ordinary imaging slice plane, it becomes a greater load to a power amplifier for RF pulses. For this reason, a power amplifier having a high output is necessary. To excite the artifact generation portions, a frequency band width must be made greater by at least ten times than for the excitation of the slice plane, and in this aspect, too, any improvement has been desired.